Process for producing high-strength hot-dip galvanized steel sheet

ABSTRACT

A high-strength hot-dip galvanized steel sheet containing a main component, the steel sheet having at least 40 wt. % of ferrite as a main phase in terms of the volumetric ratio, and 8-60% inclusive of residual austenite, the remaining structure comprising one or more of bainite, martensite, or pearlite. Austenite particles within a range where the average residual stress (sigmaR) thereof satisfies the expression −400 MPa&lt;=sigmaR&lt;=200 MPa (formula (1)) are present in an amount of 50% or more in the hot-dip galvanized steel sheet. The surface of the steel sheet has a hot-dip galvanized layer containing less than 7 wt. % of Fe, the remainder comprising Zn, Al and inevitable impurities.

TECHNICAL FIELD

The present invention relates to a high-strength (for example, a tensilestrength of 980 MPa or more) hot-dip galvanized steel sheet withexcellent bendability, which is used for an automotive structuralmaterial and the like, and a process for producing the same.

BACKGROUND ART

For the purpose of enhancing the fuel efficiency of an automobile andachieving collision safety, application of a high tensile strength steelsheet to a vehicle body frame structure is proceeding, but on the otherhand, the increase in the strength of a material involves a decrease informability of the material, and therefore, the steel sheet used may berequired to satisfy both high press workability and high strength.

In a high-strength steel sheet, a retained (or residual) austenite steelhaving retained austenite in the steel structure may be known to,despite high strength, exhibit very high elongation by making use of aTRIP effect, In order to more increase the elongation of this retainedaustenite steel, for example, Patent Document 1 discloses a technique ofensuring uniform elongation by controlling two kinds of ferrite(bainitic ferrite and polygonal ferrite) while keeping the retainedaustenite fraction high.

Meanwhile, in forming a high-strength steel sheet having a tensilestrength of 980 MPa or more, the work may be often performed mainly bybend forming but not draw forming that has prevailed in forming alow-strength steel sheet having a tensile strength of 440 MPa or less.Similarly to elongation, enhanced bendability may be required of also ahigh-strength sheet steel.

Conventionally, it has been known that V-bendability correlates withlocal ductility, and as a technique for enhancing the local ductility,Patent Document 1 discloses a technique of making the structure uniformand increasing the strength by adding a precipitation strengtheningcomponent to a ferrite single phase, and Patent Document 2 discloses atechnique of similarly making the structure uniform by a structuremainly composed of bainite.

Also, Patent Document 3 discloses a high-strength high-ductility hot-dipgalvanized steel sheet containing, in terms of volume fraction, from 30to 90% of a ferrite phase, 5% or more of bainite, 10% or less ofmartensite, and from 5 to 30% of a retained austenite phase. PatentDocument 4 discloses a high-strength cold-rolled steel sheet, where thedensity of dislocations contained in the steel sheet is 8×10¹¹(dislocations/mm²) or less, and the static/dynamic ratio (=FS2/FS1) as aratio between a quasi-static strength (FS1) at a strain rate of 0.0067(s⁻¹), and a dynamic strength (FS2) at a strain rate of 1,000 (s⁻¹) is1.05 or more.

However, at present, higher strength and higher workability are requiredof also in the case of a high-strength steel sheet, and a techniquecapable of satisfying this requirement and also of satisfying both ofthe elongation and V-bendability at a sufficiently high level is notknown.

RELATED ART Patent Documents

[Patent Document 1] JP-A (Japanese Unexamined Patent Publication; KOKAI)No. 2003-306746

[Patent Document 2] JP-A No. 4-88125

[Patent Document 3] JP-A No. 2005-133201

[Patent Document 4] JP-A No. 2002-30403

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

The present invention has been made to solve conventional problems, andan object of the present invention is to provide a high-strength hot-dipgalvanized steel sheet excellent in elongation and V-bendability, whichis a technique found from many diligent studies to enhance theV-bendability of a retained austenite steel having a tensile strength of980 MPa or more, and a production process therefor.

Means for Solving the Problem

As a result of earnest study, the present inventors have found thatincreasing the stability of retained austenite more than ever byimparting a residual compression stress to the retained austenite phasemay effectively act on the local bending deformation of the tensilestress part outside bending and the compression stress part insidebending.

As a result of further study based on the above discovery, the presentinventors have further made studies based on the finding above, as aresult, it has been found that when the roll diameter, tension andnumber of passes in repeated bending during an over-aging (OA) treatmentare optimally controlled so as to impart a residual compression stressto the retained austenite phase, a sufficiently high effect may beobtained on elongation and V-bendability. The present inventors havestill further made studies based on the finding above, as a result, ithas been found that when control of conditions in repeated bendingduring an over-aging (OA) treatment is conformed to control of theenrichment into austenite phase and the grain size, the stability ofretained austenite phase can be increased and this may be more effectivefor elongation and V-bendability.

According to the knowledge and investigations of the present inventors,the mechanism for providing the above-described effect in the presentinvention may be presumed as follows.

Thus, the retained austenite steel may be a high-strength steel sheetobtained by controlling ferrite transformation and bainitetransformation during annealing to increase the C concentration inaustenite and thereby retain austenite in the steel structure of aproduct, and thanks to TRIP effect of the retained austenite, capable ofexhibiting high elongation. However, because of a mixed structure, sucha retained austenite steel may be presumed not to be a steel excellentin bendability.

Meanwhile, the present inventors have made various studies on the methodfor obtaining desired tensile strength, ductility, V-bendability andplating property by performing, in a laboratory, melting, hot rolling,cold rolling, annealing and hot-dip galvanization of various steelschanged in the amounts of C, Si and Mn with an attempt to achieve aneffective action of TRIP effect on bendability.

As a result of these earnest study, it has been found that when not onlyvarious components effective for the above-described purpose arespecified but also a residual compression stress is imparted to theretained austenite phase, the stability of retained austenite may beincreased more than ever and at the same time, an effective action maybe exerted on the local bending deformation of the tensile stress partoutside bending and the compression stress part inside bending.

The present inventors have accomplished the present invention, based onthe above discoveries. The present invention may include, for example,the following embodiments.

[1] A hot-dip galvanized steel sheet, which is a steel sheet comprising,in mass %,

C: from 0.10 to 0.4%,

Si: from 0.01 to 0.5%,

Mn: from 1.0 to 3.0%,

O: 0.006% or less,

P: 0.04% or less,

S: 0.01% or less,

Al: from 0.1 to 3.0%,

N: 0.01% or less, and

Si+Al≧0.5%, with the balance being Fe and unavoidable impurities,

wherein the steel sheet is a high-strength hot-dip galvanized steelsheet comprising, as the main phase, in terms of volume fraction, 40% ormore of ferrite and from 8 to 60% of retained austenite, and the balancestructure is composed of any one member or two or more members ofbainite, martensite and pearlite,

wherein out of the austenite, an austenite grain having an averageresidual stress σR satisfying formula (1) accounts for 50% or more:−400 MPa≦σR≦200 MPa  (1)and

wherein the steel sheet has, on the surface thereof, a hot-dipgalvanized layer comprising Fe in an amount of less than 7 mass %, withthe balance being Zn, Al and unavoidable impurities.

[2] The hot-dip galvanized steel sheet according to [1], wherein theaverage particle size of the austenite is 10 μm or less, the average Cconcentration in the austenite is 0.7% or more to 1.5% or less in termsof mass %.

[3] The hot-dip galvanized steel sheet according to [1] or [2], whereinthe average aspect ratio with respect to the rolling direction of theaustenite grain is from 0.5 to 0.95.

[4] The hot-dip galvanized steel sheet according to [1] or [2], whereinthe steel sheet further comprises one member or two or more members of,in mass %,

Mo: from 0.02 to 0.5,

Nb: from 0.01 to 0.10%,

Ti: from 0.01 to 0.20%,

V: from 0.005 to 0.10%,

Cr: from 0.1 to 2.0%,

Ca: from 0.0005 to 0.05%,

Mg: from 0.0005 to 0.05%,

REM: from 0.0005 to 0.05%,

Cu: from 0.04 to 2.0%,

Ni: from 0.02 to 1.0%,

B: from 0.0003 to 0.007%.

[5] A process for producing a hot-dip galvanized steel sheet, comprisingsubjecting a steel material comprising, in mass %,

C: from 0.10 to 0.4%,

Si: from 0.01 to 0.5%,

Mn: from 1.0 to 3.0%,

O: 0.006% or less,

P: 0.04% or less,

S: 0.01% or less,

Al: from 0.1 to 3.0%,

N: 0.01% or less, and

Si+Al0.5%, with the balance being Fe and unavoidable impurities, to ahot rolling treatment at a hot-rolled slab temperature of 1,100° C. ormore and a finishing temperature of 850 to 970° C.,

cooling the steel sheet after the hot rolling to a temperature region of650° C. or less at 10 to 200° C./sec on average, and taking it up in atemperature range of 650° C. or less,

cold-rolling the steel sheet at a rolling reduction ratio of 40% ormore,

annealing the steel sheet by setting the maximum temperature duringannealing to be from 700 to 900° C., cooling the steel sheet to atemperature region of 350 to 550° C. at an average cooling rate of 0.1to 200° C./sec, and then holding it in the temperature region for 1 to1,000 seconds, and

immersing the steel sheet after holding in the temperature region in ahot-dip galvanizing bath and after the plating treatment, applying analloying treatment at a temperature of 470 to 580° C.,

wherein at the time of holding the steel sheet in a temperature regionof 350 to 550° C., the steel sheet is repeatedly bent using a rollhaving a roll diameter of 50 to 2,000 mm to thereby impart a strain tothe steel sheet, and

the longitudinal average stress applied to the steel sheet during therepeated bending is from 2 to 50 MPa.

[6] The process for producing a hot-dip galvanized steel sheet accordingto [5], wherein the number of passes during the repeated bending is from2 to 6.

Effect of the Invention

The present invention can provide a hot-dip galvanized steel sheethaving a high strength and being excellent in the ductility andV-bendability. The production of the hot-dip galvanized steel sheetaccording to the present invention may be relatively easy and can beperformed stably. Therefore, the hot-dip galvanized steel sheet may beoptimally usable particularly as a steel sheet for automobiles in recentyears, which is intended for attaining weight reduction. As a result,the industrial value thereof may be remarkably high.

FIG. 1 is a graph showing a relationship between the residual stress inretained austenite phase and the minimum bend radius R.

FIG. 2 is a graph showing the range where the average residual stress σRof an austenite grain satisfies formula (1).

FIG. 3 is a graph showing a relationship between the percentage ofaustenite grain satisfying formula (1) and the minimum bend radius R.

FIG. 4 is a graph showing a relationship between the average grain sizeof retained austenite and the minimum bend radius R.

FIG. 5 is a graph showing a relationship between the aspect ratio ofretained austenite grain and the minimum bend radius R.

FIG. 6 is a graph showing a relationship between the C concentration andthe minimum bend radius R.

MODES FOR CARRYING OUT THE INVENTION

The high-strength thin steel sheet of the present invention may be theresult of attention focused on increasing the stability of retainedaustenite phase in a retained austenite steel. The present invention hasbeen achieved, as described above, based on finding that by controllingthe residual stress and aspect ratio of the retained austenite phase,the stability can be increased to an extreme and all of strength,elongation and V-bendability can be satisfied at a high level.

The structure in the hot-dip galvanized steel sheet of the presentinvention must be mainly composed of a ferrite phase and a bainite phaseand contain 3% or more of a retained austenite phase. In the case ofintending to achieve a higher strength, the structure may containmartensite, but if the structure is not mainly composed of a ferritephase and a bainite phase, elongation may be likely to significantlydecrease.

The residual stress in the retained austenite phase may be one of mostimportant factors in the present invention. As shown in FIG. 1, when theresidual stress in the retained austenite phase is lower, particularly,is 15 MPa or less, the minimum bend radius R may become smallest. Aresidual compression stress may be imparted to individual retainedaustenite grains in the production process, whereby martensitetransformation during work may be retarded, as a result, the stabilityof the whole phase may be increased.

In order to achieve this effect, as shown in FIG. 2, Er may becomeminimum in the range where the average residual stress σR of theaustenite grain satisfies formula (1). Also, as shown in FIG. 3, whenthe austenite grain satisfying formula (1) accounts for 50% or more, theminimum bend radius R may stably become smallest.−400 MPa≦σR≦200 MPa  (1)

The method for measuring the percentage of retained austenite may be anymethod as long as it is a measuring method guaranteeing the accuracy,but, for example, the measurement was performed on a surface formed bychemical polishing to a ¼ thickness from the surface layer of the samplematerial sheet, and the retained austenite was quantitatively determinedfrom the integrated intensities of (200) and (211) planes of ferrite andthe integrated intensities of (200), (220) and (311) planes ofaustenite, which were measured with a mono-chromatized MoKα ray. Themethod for measuring σR may be any method as long as it is a measuringmethod where accuracy is guaranteed under the condition of exactlyobtaining a residual stress, but in the present invention, on a surfaceformed by chemical polishing to a ¼ thickness from the surface layer ofthe sample material sheet, measurement of a residual stress of arbitrary50 retained austenite grains may be carefully performed by a stressmeasuring method using a high-precision radiation X-ray diffractionhaving a beam system of 5 μmφ, and the percentage of retained austenitegrain falling in the range of (1) can be thereby obtained.

In the present invention, the average grain size of retained austenitemay be preferably 10 μm or less. As shown in FIG. 4, if the averagegrain size exceeds 10 μm, the dispersion of retained austenite phase maybe coarsened, and the TRIP effect may not be fully exerted, giving riseto reduction in elongation. Here, the method for measuring the averagegrain size (average equivalent-circle diameter) may be any method aslong as it is a measuring method guaranteeing accuracy, but, forexample, the steel sheet in a cross-section in the rolling direction orin a cross-section perpendicular to the rolling direction was etchedwith a nital reagent, and the average grain size was quantitativelydetermined by observation through an optical microscope at 500 times.

Also, as shown in FIG. 5, when the aspect ratio of the retainedaustenite grain is from 0.5 to 0.95 with respect to the rollingdirection, the minimum bend radius may advantageously become smallest.If the aspect ratio exceeds 0.95 or less than 0.5, the stability ofretained austenite may vary during bending deformation. Here, the methodfor measuring the aspect ratio may be any method as long as it is ameasuring method guaranteeing accuracy, but for example, the steel sheetin a cross-section in the rolling direction or in a cross-sectionperpendicular to the rolling direction was etched with a nital reagent,the grain size was quantitatively determined by observation through anoptical microscope at 500 times, the grain size in the rolling directionand the grain size in a direction perpendicular to rolling were measuredon 30 retained austenite grains by an image processing software todetermine the aspect ratio, and an average value thereof was taken as arepresentative value of the material.

The average C concentration of retained austenite may also greatlycontribute to stability of the retained austenite. As shown in FIG. 6,if the average C concentration is less than 0.7% in terms of mass %, thestability of retained austenite may be extremely reduced and therefore,the TRIP effect cannot be effectively exerted, resulting indeterioration of elongation. On the other hand, even if theconcentration exceeds 1.5%, not only the elongation improving effect maybe saturated but also the cost for the production may be increased. Forthis reason, the concentration may be preferably from 0.7 to 1.5%. Here,the method for measuring the C concentration may be any method as longas it is a measuring method where accuracy is guaranteed under thecondition of exactly obtaining a resolved concentration, but, forexample, the C concentration can be obtained by using a FE-SEM-attachedEPMA and carefully measuring the concentration at a pitch of 0.5 μm orless.

First, the reasons for the limitation on the components of a steel sheetare described. In this connection, “%” means mass %.

C:

C may be an element capable of increasing the strength of the steelsheet. However, if its content is less than 0.1%, it may be difficult tosatisfy both of the tensile strength of 980 MPa or more, and theworkability. On the other hand, if the content exceeds 0.40%, spotweldability can be hardly ensured. For this reason, the range of the Ccontent is set to be from 0.1 to 0.40% or less. The C content maypreferably be from 0.13 to 0.3, more preferably from 0.19 to 0.28.

Si:

Si may be an alloying (or strengthening) element and may be effective inincreasing the strength of the steel sheet. Also, this element maysuppress the precipitation of cementite and in turn, contribute tostabilization of retained austenite, and therefore, its addition may beindispensable. If its content is less than 0.01%, the effect ofincreasing the strength may be small. On the other hand, if the contentexceeds 0.5%, the workability may be reduced. For this reason, the Sicontent is set to be from 0.01 to 0.5%. The Si content may preferably befrom 0.2 to 0.5%, more preferably from 0.1 to 0.45%.

Mn:

Mn may be an alloying element and may be effective in increasing thestrength of the steel sheet. However, if its content is less than 1.0%,the tensile strength of 980 MPa or more may be difficult to obtain. Onthe other hand, if the content is large, co-segregation with P or S maybe promoted to involve significant deterioration of the workability andtherefore, an upper limit of 3.0% is specified. For this reason, the Mncontent is set to be from 1.0 to 3.0%. The Mn content may preferably befrom 1.0 to 2.8%, more preferably from 1.0 to 2.8%.

O:

O may form an oxide and deteriorate the elongation, bendability or holeexpandability and therefore, the amount added of this element must bekept low. Among others, an oxide may often exist as an inclusion andwhen the oxide is present in the punched edge face or cut surface, anotched flaw or a coarse dimple may be formed on the end face to invitestress concentration during hole expansion or severe working and serveas an origin of crack formation, giving rise to significantdeterioration of the hole expandability or bendability. If the contentof O exceeds 0.006%, the above-described tendency may be conspicuous,and therefore, the O content is specified to an upper limit of 0.006% orless. That is, O is limited as an impurity to 0.006% or less. The upperlimit of the O content may preferably be 0.005% or less, more preferably0.004% or less. On the other hand, an O content of less than 0.0001% maybe economically disadvantageous because of involving an excessive risein the cost, and therefore, this value may be substantially the lowerlimit.

P:

P may tend to be segregated at the center in the sheet thickness of thesteel sheet and bring about embrittlement of a welded part. If itscontent exceeds 0.04%, significant embrittlement of the welded part mayoccur, and therefore, a proper content range of 0.04% or less isspecified. That is, P is limited as an impurity to 0.04% or less. The Pcontent may preferably be 0.03% or less, more preferably 0.025% or less.The lower limit of P content may not be particularly specified, but acontent of less than 0.0001% may be economically disadvantageous, andtherefore, this value may preferably be set as the lower limit.

S:

S may adversely affect the weldability and manufacturability duringcasting and hot rolling. For this reason, the upper limit of its contentis set to 0.01% or less. That is, S is limited as an impurity to 0.01%or less. The S content may preferably be 0.004% or less, more preferably0.003% or less. The lower limit of S content may not be particularlyspecified, but a content of less than 0.0001% may be economicallydisadvantageous, and therefore, this value may preferably be set as thelower limit. In addition, since S may be bound to Mn to form coarse MnSand deteriorate the bendability or hole expandability, the content ofthis element should be set as low as possible.

Al:

Al may promote ferrite formation to enhance the ductility and therefore,may be added. This element may also be utilized as a deoxidizing agent.If its content is less than 0.1%, the effect of the element may beinsufficient. On the other hand, its excessive addition may lead to anincrease in the number of Al-based coarse inclusions and give rise todeterioration of hole expandability or cause a surface flaw. For thisreason, the upper limit of the Al content is set to 3.0%. The Al contentmay preferably be from 0.1 to 2.9%, more preferably from 0.15 to 2.9%.

In the present invention, (Al+Si) is set to 0.5% or more. The (Al+Si)may preferably be 0.5 to 4%, more preferably 0.51 to 3.5%

N:

N may form a coarse nitride to deteriorate the bendability or holeexpandability and therefore, the amount added thereof must be kept low.If the N content exceeds 0.01%, the tendency above may be conspicuous,and therefore, the range of the N content is set to 0.01% or less. N canbe a cause for the formation of a blow hole during welding, andaccordingly, the N content may be smaller. The N content may preferablybe 0.005% or less, more preferably 0.004% or less. Although the effectaccording to the present invention can be achieved without particularlyspecifying the lower limit, an N content of less than 0.0005% mayinvolve a great increase in the production cost, and therefore, thisvalue may be substantially the lower limit.

Mo:

Cr may be an alloying element and may be important in enhancing thequenchability. However, if its content is less than 0.02%, these effectsmay not be obtained, and therefore, a lower limit of 0.02% is specified.On the contrary, containing this element in excess of 0.5% may adverselyaffect the manufacturability during production and hot rolling, andtherefore, an upper limit of 0.5% is specified.

Nb:

Nb may be an alloying element and may contribute to increase in thestrength of the steel sheet by precipitation strengthening, fine grainstrengthening through suppressing growth of a ferrite crystal grain, anddislocation strengthening through suppressing recrystallization. If theamount added thereof is less than 0.01%, these effects may not beobtained, and therefore, a lower limit of 0.01% is specified. If thiselement is contained in excess of 0.1%, the amount of carbonitrideprecipitated may be increased to deteriorate the formability, andtherefore, an upper limit of 0.1% is specified.

Ti:

Ti may be an alloying element and may contribute to increasing thestrength of the steel sheet by precipitation strengthening, fine grainstrengthening through suppressing growth of a ferrite crystal grain, anddislocation strengthening through suppressing recrystallization. If theamount added thereof is less than 0.01%, these effects may not beobtained, and therefore, a lower limit of 0.01% is specified. If thiselement is contained in excess of 0.2%, the amount of carbonitrideprecipitated may be increased to deteriorate the formability, andtherefore, an upper limit of 0.2% is specified.

V:

V may be an alloying element and may contribute to increasing thestrength of the steel sheet by precipitation strengthening, fine grainstrengthening through suppressing growth of a ferrite crystal grain, anddislocation strengthening through suppressing recrystallization. If theamount added thereof is less than 0.005%, these effects may not beobtained, and therefore, a lower limit of 0.005% is specified. If thiselement is contained in excess of 0.1%, the amount of carbonitrideprecipitated may be increased to deteriorate the formability, andtherefore, an upper limit of 0.1% is specified. The V content maypreferably be from 0.005 to 0.4%, more preferably from 0.005 to 0.3%.

Cr:

Cr may be an alloying element and at the same time, may be important inenhancing the quenchability. However, if its content is less than 0.1%,these effects may not be obtained, and therefore, a lower limit of 0.1%is specified. On the contrary, containing this element in excess of 2.0%may adversely affect the manufacturability during production and hotrolling, and therefore, an upper limit of 2.0% is specified.

One member or two or more members selected from Ca, Mg and REM may beadded in a total amount of 0.0005 to 0.04%. Ca, Mg and REM may be anelement used for deoxidization, and it may be preferred to contain onemember or two or more members thereof in a total amount of 0.0005% ormore. Here, REM is Rare Earth Metal. However, if the total content ofCa, Mg and REM exceeds 0.05%, degradation of forming workability may becaused. For this reason, the total content thereof is set to be from0.0005 to 0.05%.

Incidentally, in the present invention, REM may be added in the form ofmisch metal in many cases, and there may be a case in which acombination of elements in the lanthanoid series is contained inaddition to La and Ce. Even when such elements in the lanthanoid seriesother than La and Ce are contained as unavoidable impurities, theeffects according to the present invention may be brought out. In thisconnection, the effects according to the present invention may also bebrought out even when metallic La and Ce are added.

Cu:

Cu may be an alloying element and at the same time, may be important inenhancing the quenchability. In addition, Cu may enhance the wettabilityof molten metal or promote an alloying reaction and therefore, may beadded. However, if its content is less than 0.04%, these effects may notbe obtained, and therefore, a lower limit of 0.04% is specified. On thecontrary, containing this element in excess of 2.0% may adversely affectthe manufacturability during production and hot rolling, and therefore,an upper limit of 2.0% is specified.

Ni:

Ni may be an alloying element and at the same time, may be important inenhancing the quenchability. In addition, Ni may enhance the wettabilityof molten metal or promote an alloying reaction and therefore, may beadded. However, if its content is less than 0.02%, these effects may notbe obtained, and therefore, a lower limit of 0.02% is specified. On thecontrary, containing this element in excess of 1% may adversely affectthe manufacturability during production and hot rolling, and therefore,an upper limit of 1.0% is specified.

Addition of B in an amount of 0.0003% or more may be effective instrengthening a grain boundary or increasing the strength of steelmaterial, but if the amount added exceeds 0.07%, not only the effect maybe saturated but also the manufacturability during hot rolling may bereduced, and therefore, an upper limit of 0.07% is specified.

Next, the structure of the steel material may be described below.

In the steel sheet according to the present invention, ferrite is usedas a main phase, and retained austenite of volume fraction of 8% or moreis dispersed therein, to thereby secure a tensile strength of 980 MPa ormore. Accordingly, the steel sheet should contain retained austenite. Asthe form of ferrite, acicular ferrite may be incorporated other thanpolygonal ferrite. The reason why ferrite is used as the main phase maybe because by forming the main phase from ferrite rich in ductility, theductility may be enhanced. If the content of the ferrite phase as themain phase is less than 40%, sufficient ductility may not be secured,and the steel sheet may not be suited for practical use. For thisreason, the volume fraction of main-phase ferrite is set to 40% or more.

Retained austenite is contained as a second phase, whereby increase instrength and more enhancement of ductility may be achieved at the sametime. If the volume fraction thereof is less than 8%, the effect abovemay be hardly obtained, and therefore, a lower limit of less than 8% isspecified. The reason why the upper limit is set to 60% or less isbecause if its volume fraction exceeds 60%, the volume fraction offerrite phase as the main phase may become less than 40%, and sufficientductility may not be secured. The bainite structure may be utilized forthe stabilization of retained austenite and therefore, may beunavoidably contained. For further increasing the strength, martensitemay be contained.

With respect to the above-described microstructure phases, ferrite,martensite, bainite, austenite, pearlite and the balance structure, theidentification, observation of existing position, and measurement ofarea ratio can be performed by using a nital reagent and a reagentdisclosed in JP-A No. 59-219473 to etch the steel sheet in the rollingdirection cross-section or the cross-section in the directionperpendicular to the rolling direction and effecting a quantitativedetermination by observation through an optical microscope at 1,000times and scanning and transmission electron microscopes at 1,000 to100,000 times. After observation of 20 or more visual fields for each,the area ratio of each structure can be determined by a point countingmethod or image analysis.

The production process for the high-strength hot-dip galvanized steelsheet with excellent plating adhesion according to the present inventionis described below.

A slab before hot rolling may be subjected to normal hot rolling aftercontinuous casting.

For example, a slab after continuous casting is set to 1,100° C. or moredirectly or through re-heating. At a temperature less than thetemperature above, insufficient homogenization may result to cause areduction in the strength and V-bendability.

Subsequently, the slab is hot-rolled at a finishing temperature of 850to 970° C. Because, if the finishing temperature is less than 850° C.,the rolling may be (α+γ) two-phase region rolling, and rollability maybe deteriorated, whereas if the finishing temperature exceeds 970° C.,the austenite grain size may be coarsened, and the ferrite phasefraction may become small, giving rise to reduction in ductility.

Thereafter, the slab is cooled to a temperature region of 650° C. orless at 10 to 200° C./sec on average, and then taken up at a temperatureof 650° C. or less. If the cooling rate is less than the range above orthe take-up temperature exceeds the range above, a pearlite phase thatsignificantly deteriorates the bendability may be produced. If theaverage cooling rate exceeds 200° C./sec, the effect of suppressingpearlite may be saturated, and the cooling endpoint temperature maysignificantly vary, making it difficult to ensure a stable materialquality. For this reason, the cooling rate is set to 200° C./sec orless.

After pickling, the sample material may be subjected to cold rolling of40% or more. If the rolling reduction is less than this range,recrystallization or reverse transformation during annealing may besuppressed to cause reduction in the elongation.

The maximum temperature during annealing is set to be from 700 to 900°C. If the maximum temperature is less than 700° C., recrystallization ofa ferrite phase during annealing may be retarded to cause reduction inthe elongation. On the other hand, at a temperature in excess of thetemperature above, the martensite fraction may be increased to causedeterioration of the elongation.

In order to freeze the structure and effectively bring out bainitetransformation during cooling after a soaking treatment in the annealingstep, the cooling rate may be preferably higher. In this connection, ifthe cooling rate is less than 0.1° C./sec, the transformation cannot becontrolled, whereas even if the cooling rate exceeds 200° C./sec, theeffect may be saturated and in addition, the temperature controllabilityof the cooling endpoint temperature that is most important for theproduction of retained austenite may be significantly deteriorated. Forthis reason, the cooling rate after annealing may be preferably from 0.1to 200° C./sec on average. The cooling rate may be, on average, morepreferably from 1.2 to 14° n/sec, still more preferably from 1.8 to 11°C./sec.

The cooling endpoint temperature and subsequent holding or leaving tocool may be an important technique to control the production of bainiteand determine the C concentration of retained austenite. If the coolingendpoint temperature is less than 350° C., a large amount of martensitemay be produced to excessively increase the steel strength and inaddition, it may be difficult to retain austenite, as a result,elongation may be deteriorated to an extremely large extent. On theother hand, if the cooling endpoint temperature exceeds 550° C., thebainite transformation may be retarded and in addition, production ofcementite may occur during holding or leaving to cool, decreasing the Cenrichment in retained austenite. For this reason, the cooling stoptemperature and the holding or leaving-to-cool temperature, whereretained austenite having a high C concentration can be produced at 8%or more, may be preferably from 350 to 550° C.

The holding or leaving-to-cool time may be preferably longer in view ofC enrichment into retained austenite. If the time is less than 1 second,bainite transformation may not sufficiently occur, and inadequate Cenrichment may result. On the other hand, if the time exceeds 1,000seconds, cementite may be produced in the austenite phase and in turn,the C concentration may be likely to decrease. For this reason, it maybe preferred to set the holding or leaving-to-cool time to be from 1 to1,000 seconds. The holding or leaving-to-cool time may be preferablyfrom 110 to 800 seconds, more preferably from 150 to 400 seconds.

The residual stress in austenite phase and the aspect ratio of retainedaustenite grain can be controlled by optimally controlling the rolldiameter, tension and number of passes in repeated bending duringholding (during an over-aging (OA) treatment), but when the over-aging(OA) treatment time is set to be from 350 to 550° C., all of thefollowing requirements must be satisfied. The roll diameter may bepreferably smaller so as to impart a certain strain. However, if theroll diameter is 50 mm or less, the roll rigidity may be decreased and astable strain cannot be imparted. On the other hand, if the rolldiameter exceeds 2,000 mm, the surface-contact area may be increased,making it impossible to locally impart a large strain. For this reason,the roll diameter may be preferably from 50 to 2,000 mm. The lower limitmay be more preferably 350 mm or more, and the upper limit may bepreferably 1,000 mm or less.

Also, the tension may be adjusted by the longitudinal average stressthat is a value obtained by dividing the tension by the sheet'scross-sectional area (sheet thickness×sheet width) and may be animportant value for determining the aspect ratio in the longitudinaldirection, but if the strength (TS) of steel sheet when the over-aging(OA) temperature is set to be from 350 to 550° C. exceeds 50%, the riskof fracture may increase. Since the strength of 980 MPa steel at 350° C.may be about 100 MPa, the tension (longitudinal average stress) may besuitably 50 MPa or less. This tension (longitudinal average stress) maybe preferably 45 MPa or less, more preferably 40 MPa or less. The lowerlimit may not be specifically specified, but considering the passabilityof steel sheet, particularly meandering, the tension may be preferably 2MPa or more, more preferably 10 MPa or more.

As for the number of passes, a larger number of passes may increase theoccurrence of bending/unbending and facilitate the control of residualstress, but the effect may be small in one pass. For this reason, 2passes or more may be usually preferred. The number of occurrences ofbending may be preferably 6 passes or less, more preferably 5 passes orless.

Thereafter, the steel sheet is immersed in a hot-dip galvanizing bath.In the technique of the present invention, an alloying treatment isperformed after the immersion. At this time, an alloying treatment ofthe plating layer is performed at 470 to 580° C. At a temperature lowerthan this range, alloying may be insufficient, whereas at a temperatureexceeding the range above, over-alloying may occur and the corrosionresistance may be significantly deteriorated.

EXAMPLES

Hereinbelow, the present invention may be described in more detail belowby referring to Examples.

A steel having the component composition shown in Table 1 was produced,cooled/solidified, then re-heated to 1,200° C., finish-rolled at 880°C., cooled, further cooled to 550° C. at an average cooling rate of 60°C./sec, and taken up at the take-up temperature shown in Table 2.Thereafter, the resulting hot-rolled sheet was subjected to cold rollingof 50% and then annealed by continuous annealing under the conditionsshown in Table 2.

Assuming the effects of roll diameter, tension and number of passes inrepeated bending during an over-aging (OA) treatment in an actualproduction line, a plurality of different curvatures, tensions andoccurrence numbers were given during OA of the annealing treatment, andthe effect on the residual stress was evaluated.

TABLE 1 No. of Steel Species C Si Mn O P S N Al Al + Si Others 1 0.120.25 2.7 0.005 0.01 0.002 0.004 0.25 0.50 — 2 0.1 0.25 2.4 0.003 0.0120.003 0.0033 0.28 0.53 — 3 0.12 0.45 1.62 0.001 0.011 0.004 0.0043 2.783.23 Cr: 1.26 4 0.15 0.02 2.5 0.001 0.013 0.004 0.0022 1.63 1.65 Ce:0.01, La: 0.002, V: 0.4 5 0.35 0.30 1.6 0.002 0.012 0.01 0.0022 1.842.14 — 6 0.27 0.10 2.45 0.004 0.01 0.0013 0.0024 2.53 2.63 Mg: 0.0008 70.32 0.28 2.5 0.004 0.02 0.0023 0.0029 0.22 0.50 Ca: 0.008 8 0.38 0.272.4 0.001 0.01 0.0014 0.0034 2.51 2.78 — 9 0.15 0.25 1.9 0.005 0.020.002 0.0041 1.86 2.11 Ti: 0.01 10 0.19 0.23 1.7 0.003 0.03 0.001 0.0021.47 1.71 B: 0.001 11 0.18 0.22 1.8 0.002 0.02 0.002 0.0024 2.88 3.10Mo: 0.1 12 0.2 0.30 2.6 0.002 0.03 0.001 0.0033 0.26 0.56 Cr: 0.8 130.194 0.28 2.5 0.002 0.013 0.0015 0.0012 0.23 0.52 Nb: 0.051 14 0.210.25 2.4 0.002 0.006 0.0042 0.0043 0.59 0.84 Ti: 0.056, B: 0.0053 150.19 0.30 1.82 0.002 0.011 0.0032 0.0027 0.69 0.99 Mo: 0.33 16 0.7 0.402.3 0.002 0.013 0.0047 0.0039 0.42 0.82 — 17 0.22 0.57 2.5 0.004 0.0140.0037 0.0015 0.47 1.04 — 18 0.1 0.33 3.5 0.004 0.014 0.0049 0.0012 2.212.54 Ca: 0.015 19 0.12 0.32 2.5 0.01 0.13 0.033 0.001 2.50 2.82 20 0.140.37 2.4 0.002 0.08 0.0015 0.005 2.90 3.27 — 21 0.19 0.33 2.8 0.0020.011 0.06 0.001 0.51 0.85 — 22 0.28 0.23 2.8 0.001 0.001 0.0015 0.0210.52 0.76 Mg: 0.0007 23 0.19 0.29 2.64 0.004 0.08 0.0015 0.005 13.9514.24 Ca: 0.003 24 0.35 0.02 2.4 0.003 0.08 0.0015 0.005 0.12 0.14 —

Thereafter, the resultant steel sheets were immersed in a galvanizingbath controlled to predetermined conditions, and the steel sheets werecooled to room temperature. At this time, the effective Al concentrationin the galvanizing bath was set to 0.09 to 0.17 mass %. With respect tosome of these steel sheets, they were immersed in a galvanizing bath,then were subjected to each of the alloying treatments under apredetermined condition therefor, and were cooled to room temperature.Finally, the thus obtained steel sheet was skin-pass rolled at a rollingreduction ratio of 0.4%.

Heat treatment conditions and plating treatment conditions are shown inTable 2.

The tensile properties were evaluated by pulling a JIS No. 5 tensiletest piece in C direction. As for the identification of structure,observation of existing position and measurements of average grain size(average equivalent-circle diameter) and occupancy, the steel sheet in across-section in the rolling direction or in a cross-sectionperpendicular to the rolling direction was etched with a nital reagent,and quantitative determination was made by observation through anoptical microscope at 500 to 1,000 times.

As for the V-bending property, a test was performed based on JIS Z 2248,and after performing the test for punch R of 0.5 mm, 1 mm and 2 mm, thesteel sheet was observed with an eye and judged as follows. “A” wasaccepted.

A: No cracking.

B: Slight cracking (a plurality of cracks were generated on the outersurface of bending).

C: Cracking occurred.

TABLE 2 No. of Take-Up Cold Rolling Annealing Temporary Primary TestSteel Temperature Reduction Temperature Cooling Cooling Rate No. Species[° C.] [%] [° C.] Temperature [° C.] [° C./sec] Remarks a 1 500 50 950678 1.2 Steel of Invention b 2 690 55 830 690 1.9 Steel of Invention c 3550 57 840 735 3.4 Steel of Invention d 4 400 49 740 740 1.8 Steel ofInvention e 5 400 69 810 719 2.9 Steel of Invention f 6 500 48 800 68413.5  Steel of Invention g 7 600 49 830 699 10.8  Steel of Invention h 8550 50 850 705 16.4  Steel of Invention i 9 630 50 840 695 2.5 Steel ofInvention j 10 620 50 750 740 2.4 Steel of Invention k 11 660 45 760 7051.9 Steel of Invention l 12 550 60 800 710 10.9  Steel of Invention m 13530 50 840 730 5.4 Steel of Invention n 14 560 50 850 700 8.2 Steel ofInvention o 15 600 50 860 720 6.2 Steel of Invention p 16 500 60 810 7291.9 Comparative Steel q 17 600 50 810 658 1.8 Comparative Steel r 18 65050 840 690 2.5 Comparative Steel s 19 600 50 850 688 2.7 ComparativeSteel t 20 610 50 850 678 3.4 Comparative Steel u 21 540 50 850 699 1.6Comparative Steel v 22 680 50 850 725 2.7 Comparative Steel w 23 350 50850 734 10.9  Comparative Steel x 24 390 50 850 719 11.3  ComparativeSteel aa 4 500 50 850 746 11.4  Comparative Steel ab 4 500 90 850 72012.5  Comparative Steel ac 4 500 50 850 670 10.7  Comparative Steel ad 4500 50 850 600 1.5 Comparative Steel ae 4 500 50 850 740 38.1 Comparative Steel af 4 500 50 850 740 1.8 Comparative Steel Conditionsof Repeated Bending During Holding Holding Tension Time at Longitudinal-Number of Retained Test 350 to Bending Average Occurrences PlatingFerrite Austenite Martensite No. 550° C. R [nm] Stress [MPa] of BendingTreatment [%] [%] [%] Remarks a 150 900 35 5 GI 52 10 3 Steel ofInvention b 180 800 25 4 GI 53  9 2 Steel of Invention c 170 400 35 5 GA56 11 3 Steel of Invention d 120 1200 23 6 GA 55 12 2 Steel of Inventione 190 1400 35 3 GI 52  9 3 Steel of Invention f 400 1300 70 4 GI 55  8 4Steel of Invention g 300 900 30 5 GI 53 10 5 Steel of Invention h 260700 30 5 GA 50 12 6 Steel of Invention i 180 100 15 4 GA 55 14 1 Steelof Invention j 190 200 35 4 GA 52 15 3 Steel of Invention k 250 1300 451 GA 49 11 3 Steel of Invention l 280 2600 50 6 GA 47 10 4 Steel ofInvention m 800 1200 50 6 GA 45 10 5 Steel of Invention n 260 1400 40 5GA 50  9 2 Steel of Invention o 300 400 35 3 GA 52 12 5 Steel ofInvention p 120 1500 10 1 GI 33  0 25 Comparative Steel q 130 1400 30 1GA 53  4 4 Comparative Steel r 250 1300 12 6 GI 29  3 21 ComparativeSteel s 270 900 10 8 GA 20 10 20 Comparative Steel t 340 800 32 5 GI 37 4 2 Comparative Steel u 370 300 23 4 GA 35  2 12 Comparative Steel v400 1500 15 1 GI 39  2 22 Comparative Steel w 250 2600 4 6 GA 45 10 3Comparative Steel x 260 2700 65 6 GI 43 11 1 Comparative Steel aa 160900 22 2 GI 40  9 2 Comparative Steel ab 145 800 20 3 GA 44 12 2Comparative Steel ac 178 450 40 1 GI 49 11 3 Comparative Steel ad 162500 15 5 GA 51  8 4 Comparative Steel ae 250 1600 10 1 GI 52 12 2Comparative Steel af  90 500 35 1 GA 55  5 2 Comparative Steel AverageResidual Grain Stress of Average C Size of Retained γ Test BainitePearlite TS EL λ Concentration Retained (average) No. [%] [%] [MPa] [%][%&] TS × EL TS × λ in Retained γ γ [μm] MPa Remarks a 26 9  987 20 5919740 58233 0.8 8 −110 Steel of Invention b 34 2 1168 19 48 22192 560640.9 10  −80 Steel of Invention c 21 9 1186 18 49 21348 58114 1.2 7  85Steel of Invention d 22 9 1239 20 45 24780 55755 1.1 7 −250 Steel ofInvention e 29 7 1379 13 30 17927 41370 0.7 8  −20 Steel of Invention f20 13 1480 12 32 17760 47360 0.8 7  50 Steel of Invention g 22 10 102124 51 24504 52071 0.9 8   8 Steel of Invention h 10 22 1450 12 31 1740044950 1.1 6 −200 Steel of Invention i 24 6 1185 17 45 20145 53325 1 8−350 Steel of Invention j 30 0 1205 16 46 19280 55430 0.9 9 −220 Steelof Invention k 21 16  989 23 50 22747 49450 0.8 9  −50 Steel ofInvention l 32 7 1201 17 35 20417 42035 0.7 9  −75 Steel of Invention m28 12 1186 19 39 22534 46254 1.2 9  55 Steel of Invention n 22 17 120817 34 20536 41072 1.1 10  −27 Steel of Invention o 23 8 1226 16 32 1961639232 0.9 8  −80 Steel of Invention p 21 21 1550 5 18 7750 27900 0 20 120 Comparative Steel q 27 12 1264 10 15 12640 18960 0.5 12  150Comparative Steel r 22 25 1197 12 13 14364 15561 0.6 13  280 ComparativeSteel s 39 11 1201 10 8 12010 9608 0.65 14 −430 Comparative Steel t 2829 1259 11 7 13849 8813 0.34 14 −250 Comparative Steel u 34 17  925 1510 13875 9250 1.6 15 −467 Comparative Steel v 22 15  945 11 19 1039517955 1.9 16 −370 Comparative Steel w 20 22  884 10 29 8840 25636 2 20 230 Comparative Steel x 23 22 1387 7 27 9709 37449 2.2 19 −450Comparative Steel aa 29 20 1184 17 25 20128 29600 2.2 12  300Comparative Steel ab 35 7  584 39 50 22776 29200 2.7 19  198 ComparativeSteel ac 29 8 1480 12 45 17760 66600 2.53 18 −430 Comparative Steel ad28 9  785 35 39 27475 30615 0.4 19 −390 Comparative Steel ae 30 4 128011 40 14080 51200 0.5 19  300 Comparative Steel af 22 16 1320 6 30 792039600 0.6 15  130 Comparative Steel Percentage (%) of RetainedV-Bendability Test γ Having Residual Stress Aspect Bending BendingBending No. Satisfying Formula (1) Ratio R: 2.0 R: 1.0 R: 0.5 Remarks a70 0.77 A A A Steel of Invention b 85 0.75 A A A Steel of Invention c 640.85 A A A Steel of Invention d 66 0.92 A A A Steel of Invention e 750.88 A A A Steel of Invention f 93 0.79 A A A Steel of Invention g 950.69 A A A Steel of Invention h 65 0.71 A A A Steel of Invention i 510.59 A A A Steel of Invention j 77 0.79 A A A Steel of Invention k 910.84 A A A Steel of Invention l 91 0.92 A A A Steel of Invention m 560.9 A A A Steel of Invention n 66 0.88 A A A Steel of Invention o 810.72 A A A Steel of Invention p 31 0 A B C Comparative Steel q 48 0.43 AA B Comparative Steel r 21 0.44 A B C Comparative Steel s 15 0.32 A A BComparative Steel t 35 0.98 B C C Comparative Steel u 15 0.97 B C CComparative Steel v 25 0.99 A A C Comparative Steel w 39 0.45 A A BComparative Steel x 10 0.35 A A C Comparative Steel aa 29 0.12 B B CComparative Steel ab 49 0.44 A B C Comparative Steel ac 33 0.98 A A BComparative Steel ad 44 0.96 A A B Comparative Steel ae 43 0.95 A C CComparative Steel af 43 0.96 A B C Comparative Steel

The method for measuring the percentage of retained austenite wasperformed on a surface formed by chemical polishing to a ¼ thicknessfrom the surface layer of the sample material sheet, and the retainedaustenite was quantitatively determined from the integrated intensitiesof (200) and (211) planes of ferrite and the integrated intensities of(200), (220) and (311) planes of austenite, which were measured with amono-chromatized MoKα ray.

The method for measuring the residual stress (σR) of the retainedaustenite phase was performed on a surface formed by chemical polishingto a ¼ thickness from the surface layer of the sample material sheet,and the average of 10 points was determined using a high-resolutionX-ray diffractometer. The high-resolution X-ray diffractometer used inthis test was D8 DISCOVER Hybrid Super Speed Solution manufactured byBruker AXS K.K. Using the strain (εR) determined from the diffractionplane spacing distribution recorded by the diffractometer and theYoung's modulus (E) of steel material, the residual stress (σR) may beobtained according to the following formula (2):σR=εR×E  (2)

Test Nos. “a” to “o” are Examples of the present invention, where allproperties passed and a steel sheet having target properties wasobtained. On the other hand, in Test Nos. “p” to “ag” where thecomponent or production process is outside the scope of the presentinvention, any of the properties failed.

INDUSTRIAL APPLICABILITY

According to the present invention, a high-strength hot-dip galvanizedsteel sheet excellent in the elongation, and V-bendability may beprovided. The production of the high-strength hot-dip galvanized steelsheet may be relatively easy and can be performed stably. Therefore, thehigh-strength hot-dip galvanized steel sheet according to the presentinvention may be optimal particularly as a steel sheet for automobilespursuing weight reduction in recent years, and its industrial value maybe remarkably high.

The invention claimed is:
 1. A process for producing a hot-dipgalvanized steel sheet, comprising subjecting a steel materialcomprising, in mass %, C: from 0.10 to 0.4%, Si: from 0.01 to 0.5%, Mn:from 1.0 to 3.0%, O: 0.006% or less, P: 0.04% or less, S: 0.01% or less,Al: from 0.1 to 3.0%, N: 0.01% or less, and Si+Al≧0.5%, with the balancebeing Fe and unavoidable impurities, to a hot rolling treatment at ahot-rolled slab temperature of 1,100° C. or more and a finishingtemperature of 850 to 970° C., cooling the steel sheet after the hotrolling to a temperature region of 650° C. or less at 10 to 200° C./secon average, and taking it up in a temperature range of 650° C. or less,cold-rolling the steel sheet at a rolling reduction ratio of 40% ormore, annealing the steel sheet by setting the maximum temperatureduring annealing to be from 700 to 900° C., cooling the steel sheet to atemperature region of 350 to 550° C. at an average cooling rate of 0.1to 200° C./sec, and then holding it in the temperature region for 1 to1,000 seconds, and immersing the steel sheet after holding in thetemperature region in a hot-dip galvanizing bath and after the platingtreatment, applying an alloying treatment at a temperature of 470 to580° C., wherein at the time of holding the steel sheet in a temperatureregion of 350 to 550° C., the steel sheet is repeatedly bent using aroll having a roll diameter of 50 to 2,000 mm to thereby impart a strainto the steel sheet, and the longitudinal average stress applied to thesteel sheet during the repeated bending is from 2 to 50 MPa.
 2. Theprocess for producing a hot-dip galvanized steel sheet according toclaim 1, wherein the number of passes during the repeated bending isfrom 2 to 6.